1. Technical Field
The present disclosure concerns a read/write transducer for a storage medium, in particular of a ferroelectric material, and an associated storage device and method, in particular for “probe storage” applications, to which the following description will make explicit reference without this implying any loss of generality.
2. Description of the Related Art
As is known, storage systems using a technology based on magnetism, such as hard disks, suffer from severe limitations regarding increases in data storage capacity and read/write speed, and size reduction. In particular, there is a physical limit, the so-called “superparamagnetic limit,” which prevents the reduction in size of magnetic storage domains under a critical threshold, below which the stored information can be lost.
Alternative storage systems have thus been proposed in recent years, amongst which the so-called “probe storage” systems (also known as “atomic storage” systems) have assumed particular significance. These systems allow high data storage capacity to be achieved with reduced size and low manufacturing costs. In brief, these storage systems comprise a two-dimensional array of transducers (or probes), each provided with a respective read/write head, located above a storage medium and movable with respect to the storage medium. Each transducer is configured to locally interact with a portion of the storage medium (corresponding to a memory track), for reading/writing individual bits of information (“1” or “0”).
In particular, in known storage systems of the “probe storage” type using a storage medium of ferroelectric material, the reading/writing of the individual bits is carried out by interacting with the ferroelectric domains of the ferroelectric material.
As is known, a ferroelectric material has a spontaneous polarization, which can be inverted by an applied electric field; furthermore, as shown in FIG. 1, this material has a hysteresis cycle. When a positive voltage V greater than a positive threshold Vth is applied to the medium of ferroelectric material, a positive charge Q is stored in the material; conversely, a negative charge −Q is stored in the material when the applied voltage drops below a negative threshold −Vth. Any intermediate voltage value between the negative and positive thresholds does not cause any change in the stored charge.
In detail, during the write operation, the ferroelectric storage medium is polarized at a reference potential (e.g., ground). Then, a write voltage Vs (FIG. 2a) having a substantially square waveform between a maximum positive value Vmax, higher than the positive threshold Vth, and a minimum negative value Vmin, lower than the negative threshold −Vth, is applied to a write head that moves above the medium. In this way, a charge sequence Qi is stored inside the storage medium, the polarity of which substantially replicates the polarity of the applied write voltage Vs (FIG. 2b). In particular, for each specified interval Tbit of this square waveform, there is the corresponding polarization of a ferroelectric domain located along a memory track, and consequently a memory cell having a high (“1”) or low (“0”) binary value according to the polarity of the stored charge.
During the read operation of previously stored information, a read head moves above the ferroelectric storage medium along the track, in contact with the ferroelectric material. The read head is polarized, for example, with a voltage greater than the positive threshold Vth. When the read head is over a domain with a negative charge, it causes the polarity inversion of the stored charge. This inversion implies an exchange of charge between the read head and the storage medium and the presence of a current I emitted by the head (FIG. 2c). The value of the current I depends on the quantity of charge stored in the cell (in turn dependent on the type of ferroelectric material) and is directly proportional to the head scanning speed. Conversely, when the read head is over a domain with a positive charge, the head does not exchange charges with the medium and therefore no appreciable current flows through the head. The read operations can therefore be carried out by a suitable circuit configured to detect the current (particularly the presence or absence of an appreciable amount of current) flowing between the read head and the storage medium. In an altogether similar manner, a polarization voltage lower than the negative threshold −Vth can be applied to the read head, such as to cause polarity inversion in the domains with a positive charge.
The main problem of this read/write scheme is tied to the fact that the read operations are destructive, i.e., they entail the removal of the stored information and therefore the impossibility of making a subsequent reading of the same data. In fact, the reading of a portion of the memory (or track) corresponds to the writing of a sequence of charges that are all positive (or all negative, if a negative polarization voltage is used for the read head) in that portion of memory. Consequently, during reading the flow of read data must be stored in a memory buffer, the dimensions of which must be at least equal to the size of the track read. Furthermore, the contents of the memory buffer must be successively rewritten to the previously read track, so that the head must be repositioned at the start of the track, must rewrite the entire track and only afterwards can it undertake a new read operation. Thus, not only is the presence of a memory buffer of large dimensions necessary, but the rate of transferring data (data rate) is greatly limited as well, in particular being at least halved (as every read operation entails a successive write operation).